Determination of the modes of progress of the definitive bile ducts. This process involves, according to the literature, transformation of hepatocytes into ductal epithelium. It also involves, according to our preliminary observations, acquisition of microvilli by ductal epithelium. METHODS: On the light microscopic level: Serial sectioning of embryos; three-dimensional reconstructions from these serial sections. On the EM level: Determining the changes which cells undergo during these processes: 1. What changes do liver cells undergo when they are transformed into epithelial cells bearing microvilli? 2. What changes does the ductal epithelium undergo when it acquires microvilli? 3. Do the two kinds of cells exert mutual influences upon one another? OBJECTIVES: To contribute to understanding the origins of neonatal biliary atresias.